Can Grape Phylloxera Activity Damage Resistant Rootstocks?

This study had two primary objectives; the first was to gather evidence of damage to
resistant rootstocks and determine cause. In order to do so, we identified problematical
vineyard blocks and ascertained moderate phylloxera activity. Cane growth was
depressed and no other likely causes of vine damage were obvious. We selected four
vineyard blocks to investigate in detail, two on Teleki 5C rootstock and two on 101-14
Mgt. Nodosities counts on feeder roots were too variable for population estimates so we
used 2-liter soil emergence traps placed on the berm adjacent vine trunks to index
phylloxera timing and abundance. Populations were highly variable through the summer
but not extremely high. We found no phylloxera on mature storage roots, i.e.
tuberosities. Nematode counts were not well associated with damage. However, fungal
virulence in soil as determined by excised root bioassays was. We documented vine
damage by pruning weights in winter and found strong differences between the damaged
vines and undamaged.
The second object was to determine the mechanism of the field damage. We
hypothesized three possible mechanisms for phylloxera to damage resistant rootstocks.
Our first hypothesis was that a new phenotype might aggressively feed on rootstock
storage roots (i.e. tuberosities). This mechanism requires numerous vine-damaging
tuberosities. We searched each site but found none disproving this hypothesis.
However, our finding of nodosities suggested some change in the phylloxera. We used a
bioassay to test various rootstocks as well as V. vinifera collecting data on population
increase. Phylloxera collected from two 101-14 Mgt sites were virulent on young feeder
roots of a number of strongly resistant rootstocks showing greater virulence than the
common ?biotype A? and ?biotype B.? This suggests that these insects were changed
from previous forms in Napa and Sonoma Counties. Twelve microsatellite primers were
used to quantify genetic strains.
Our second hypothesis was that above ground damage may be due to very highly infested
feeder roots (i.e. nodosities). Populations of phylloxera were not high enough to support
this hypothesis.
Our third hypothesis was that Phylloxera normally growing on immature roots may
wander to storage roots in search of new feeding sites. Whether they successfully
establish or not, they may spread secondary pathogens by their probing (i.e. tasting)
activity. This mechanism suggests that probing of phylloxera spreads fungal infections
which severely damages the root system and is responsible for the above ground damage
seen. Presence of phylloxera nodosities as a source of the wandering insects and the high
fungal virulence observed at damaged sites supports this hypothesis. We did enclosure
experiments with excised roots in the laboratory and intact roots in vineyards to attempt
to disprove this possibility. Roots were treated with non-virulent eggs and virulent fungi
to determine whether fungal spread by probing occurred. The results suggested that
probing occurred, though at a low rate, and therefore we could not disprove this
Continued research is needed to determine that fungal necrosis is associated with the
observed damage and that probing is the likely cause. Additionally, we need to ascertain
whether other sites are involved and the spread of the problem.